To PLC or not to PLC

July 1, 2000
That's the question for motion system engineers. PLC manufacturers continue to make their products smaller, smarter, and more open, while others introduce alternative control products and systems. Here's a look at recent developments

The PLC market should grow from $1.2 billion to over $1.4 billion during the next five years, say reports from Automation Research Corp. (ARC). This is not the level of growth this device is used to. The huge installed base accounts for some of the slow growth, but another factor is all the challengers to the PLC, such as personal computers and software versions of PLCs.

To counteract these challengers, PLC manufacturers continue to:

• Shrink the size of the PLC and expand its capability. It usually takes only a few years for features previously found in the large PLCs to show up in smaller versions.
• Unbundle the PLC. Along with I/O, other cards now operate remotely from the PLC CPU.
• Keep pace with the competition and offer their own alternative control options, such as PC-based control, soft-logic PLCs, and communication network and bus options.

The latest size of PLC introduced at recent trade shows is the nano PLC. It’s a control with ten I/O points, often expandable to 32 in the base unit. With expansion I/O chassis, it can handle over 100 I/O points. Its size is so small, 3 x 4 x 2 in., it can fit in your shirt pocket.

The size lets engineers embed it into existing control panels, an increasingly important consideration in device selection. Users do not want devices that require large enclosures or that force them to redesign equipment to accommodate an automation device.

Microprocessors facilitate the unbundling of the PLC, which also helps reduce the footprint size. With more powerful microprocessors, servo control modules, PID, communication, as well as I/O modules frequently operate closer to the process and remote from the PLC CPU. Thus, today’s PLC is often just a CPU card and power supply. Connections to any combination of the above modules comes through a communication cable.

Soft-logic PCs were introduced by entrepreneurs about two years ago. Now, major PLC manufacturers are entering the market with soft-logic versions of PLCs. Like the PC versions, these are also based on Windows NT. (For more information about soft-logic PCs, see “Tips on interfacing PC-based control into motion systems,” p. 37, in this issue.)

Alternative controls

Be it a soft or hard device, the PCbased control market is expected to grow over 150% annually for the next few years, according to reports from ARC. PC-based controls replace the proprietary PLC CPU with standard processors (such as Intel’s Pentium chip) and typically run Windows software.

Despite the introductions of smaller PLCs, (at a user cost of $200 to $300) market forecasters see PCs moving into markets these small PLCs once dominated. PCs also continue to encroach on large PLC markets.

The market’s desire for open systems, initiated by the automotive manufacturers, is a stimulus for all the partnerships and alliances between PLC manufacturers and other automation suppliers. One of the benefits of these get-togethers is that users have access to products that ease communications and connection to automation products, almost regardless of vendor. The PLC manufacturer or a third-party partner develops the connection, replacing the code and drivers that users previously created. These connections often come in the form of plug-in boards and more functional communications buses and networks.

Because of these developments, though, another alternative control is emerging. Its benefit is that it offers a way to reduce the number of controllers needed in many applications.

Reducing the number of controllers

Microprocessors enable unbundled modules to communicate over a network or bus. The network or bus effectively spreads control to where it’s needed. Therefore a control is no longer needed at every production point. An application that once required two or three controls can now be handled with one. Because the control tasks are spread among intelligent modules, the one control can be used more efficiently.

The best network for reducing the number of controls is a network based on a new communication architecture called producer-consumer. This network operates with a type of control that has a passive backplane. The passiveness of the backplane allows it to work with devices from different manufacturers. Various modules, such as PLCs, PCs, and network CPUs; motion controllers; individual servo cards; I/O modules; and so on, plug into this passive backplane. It functions as a gateway, transferring signals among the various modules plugged into it. The backplane, in effect, becomes the network. Networks based on this communication model are ControlNet, DeviceNet, and Foundation Fieldbus.

In a producer-consumer network, a node produces data that it broadcasts to any and all nodes on the network. Any node that needs the data consumes it, without waiting for a token, without waiting for a query from the control.

For example, to integrate two motion controls now, an engineer must take an encoder input and feed it to two separate input modules, which are connected to two interface modules that are connected to two separate controls. In a producer- consumer network scheme, the encoder sends data to the passive backplane through one communication cable, broadcasting them once. The backplane routes these data to any and all modules that need them. These modules can be the input cards of two servo control modules, a control and a secondary motion card, or one or more controls plugged into the passive backplane. Engineers do not need to duplicate the interface for two controllers, nor do they require two controllers. Depending on the intelligence of the cards, one control may be all that’s needed in the system.

In addition, because data are received by multiple nodes at the same time, this type of network handles applications requiring synchronization to maintain velocity or the speed relationship among axes. These applications include drive systems or web processes, and steel or paper making. Up to 32 axes can be synchronized.

Motion cards can communicate to each other for point-to-point integration without going through a control. When a motion card broadcasts a reference or position, a secondary motion card simultaneously receives it. The two cards would then do multiaxis interpolation across the network backplane.

Putting control and motion in the same chassis also facilitates system integration.

A producer-consumer communication scheme requires less interlocking between controllers, again because controllers see data simultaneously. Along with eliminating a second or third controller, such an arrangement also lets engineers eliminate the software and programs required to maintain that second or third controller.

Another example of this trend of tightly integrating only those pieces and functions needed for motion control is CTnet. It is an integrated system of motion controllers, I/O modules, and communications interfaces. Users choose the system controller, the communication medium, and the communication protocol. The system controller need not be a PLC.

This network handles those applications that require small drive systems, for example, extruders, wire drawing, small web process lines, winders, conveyor systems and so on. These applications typically require between 2 and 30 coordinated drives in a distributed control scheme.

The drive products have 32-bit RISC processors in them. The system also includes remote I/O and memory for program storage.

Other communication developments

Several companies introduced modules that provide plug-in Ethernet connections to more devices. These modules plug into the backplane of a PLC or PC. Other companies are exploring the use of the Internet as a network for the top layer of the manufacturing hierarchy.

It is already possible to use the Internet to send data on manufacturing operations to management computers. However, most companies that are doing this are not using the entire, global Internet. They are using smaller, company-wide versions known as intranets. If a facility uses Ethernet as a manufacturing backbone, with the purchase of a few software programs, users can create a company intranet. The benefits are more available information for better decision making, better connection between management and factory systems, and the potential to reduce the overall number of networks in a plant.

Towards simpler programming

Flow chart programming is becoming the standard method of programming in PC-based control. Several companies are finding ways to incorporate it into ladder logic systems, which helps make motion programming in ladder logic easier. The flow chart elements, such as If-then-else and While commands, are added as

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